This application claims the priority of German Patent Documents Nos. 101 16 478.5 and 102 05 657.9, filed Apr. 3, 2001 and Feb. 12, 2002, respectively, the disclosures of which are both expressly incorporated by reference herein.
The invention relates to a method of producing a tube-shaped torsion-proof and bending-resistant drive shaft consisting of a fiber composite material which consists at least of several wound-up webs of prepreg, the prepreg webs being wound on a withdrawable winding spindle and the tube-shaped drive shaft wound of layers of prepreg webs being heated and hardened.
Numerous systems and machines have a drive and the torque existing on the output side has to be transmitted by means of a transmission line to the operating device. The operating device may consist of various types, for example, a wheel, a roller, a propeller, a rotor and others. The operating device is not limited to a certain type. A transmission line is considered which has at least one drive shaft. In addition, gears or other force transmission devices can be used. As a rule, the drive shaft is made of a metallic material and its geometry is dimensioned corresponding to the driving task.
In the following, a drive shaft with an unchanged arrangement geometry is considered whose relatively high nominal rotational speed has to ensure a sufficient power transmission but must nevertheless be below a critical rotational speed, and which therefore, in its application, must have a relatively high resistance to bending and torsion and must at the same time be tolerant with respect to damage in regard to the torsional moment.
The demand for tolerance to damage of an undercritical drive shaft is made for reasons of personal or technical safety or because of high investment costs. Tolerance with respect to damage demands that a defined damage to the drive shaft, for example, a mechanical impact onto the drive shaft of an intensity of from 25 to 50 joule, has to be withstood by the drive shaft in a function-maintaining manner (2.5 kg from a height of fall of 1 m corresponds to 25 joule). When damage tolerance stability exists, the hidden or visible damage must not endanger the resistance to torsion of the drive shaft. The demand for tolerance to damage is required for a drive shaft which is arranged in an exposed (unencapsulated) manner with respect to the environment and may, for example, be subjected to stone throw. This requirement exists, for example, for the drive shaft of the rear rotor of a helicopter.
A drive shaft with a nominal rotational speed below the critical rotational speed is called an xe2x80x9cundercritical shaftxe2x80x9d. Such an undercritical shaft will be considered in the following.
In order to achieve a high bending resistance, the mass of the drive shaft could be increased. This would have a favorable effect with respect to the tolerance to damage. The result would be an approximation between the nominal rotational speed and the critical rotational speed. The critical rotational speed corresponds to a bending-critical rotational speed of the drive shaft, as described in D. Beitz, K. -H. Grote, Dubbelxe2x80x94xe2x80x9cTaschenbuch fxc3xcr den Maschinenbauxe2x80x9d (xe2x80x9cMechanical Engineering Manualxe2x80x9d) 19th, completely revised edition, Springer Publishers, 1997, Page B40, Section 4.1.5. This is undesirable. Nevertheless, while its bending resistance is high, the drive shaft should have a small mass, that is, be relatively thin, for reasons of a light construction. These are contrasting technical requirements. Under this aspect, such a drive shaft is constructed as a hollow shaft in order to minimize the mass.
In the case of this drive shaft, it is also not intended to lower the nominal rotational speed in order to improve the tolerance to damage by increasing the mass and in order to simultaneously operate as an undercritical shaft.
In numerous applications in machine and system construction, there is the demand for a light construction in order to nevertheless ensure a high resistance to bending and tolerance to damage in the case of an undercritical drive shaft.
In order to eliminate the disadvantage of a large mass, it is also known to produce the drive shaft of fiber composite material.
In a winding process, a filament yarn preimpregnated with resin is wound up while traversing the yarn between two spaced distance marks of a winding spindle. The term xe2x80x9ctraversingxe2x80x9d originating from the textile industry relates to the guiding of a yarn, which is to be wound up, by means of a yarn guide in the axial direction along the winding body (winding spindle) between the two spaced distance marks. In this case, several layers of yarns are formed. Subsequently, the wound yarn layers are hardened. The resin and the hardener form the so-called matrix material.
The literature reference Dr. O. Schwarz, xe2x80x9cGlasfaserverstxc3xa4rkte Kunststoffexe2x80x9d (xe2x80x9cGlass-Fiber-Reinforced Plastic Materialsxe2x80x9d) Vogel Publishers, Wxc3xcrzburg 1975, Pages 83-90, describes the production of tube-shaped components by winding technical fibers, such as glass fibers and/or carbon fibers. The term xe2x80x9cfiberxe2x80x9d is a filament fiber, that is, a yarn. Technical fibers are those materials which are not natural fibers, such as wool or cotton.
It is demonstrated that the required winding method is generally dependent on the geometry of the tube-shaped component to be wound, its desired construction characteristics and the mechanical loads to be expected later.
The selection of the process steps during the winding of a tube-shaped component will differ according to the usage purpose of the tube-shaped component. According to the above-mentioned technical literature of Dr. Schwarz, page 89, the winding of filaments or of rovings is in the foreground. By means of the winding process, it is possible to provide the tube with resistance to bending and torsion.
However, this type of a tube, which is produced by means of a winding process of a filament or of a roving, does not meet the requirements with respect to a drive shaft which is tolerant with respect to damage.
Also, the production of individual woven hoses, their arrangement inside one another, the impregnating with a resin and a hardener and the subsequent hardening are known for producing a drive shaft. It is considered to be problematic that a complete reproducibility of the quality characteristics cannot be ensured without high expenditures. The insertion of the woven hoses results in the risk that the fiber orientation may be undesirably changed and is not recognized. As a result, no satisfactory drive shaft is achieved which is tolerant to damage.
Generally, a prior art method of producing a drive shaft is illustrated in FIGS. 1A-1C. As shown in FIG. 1A, a first process step 10 includes winding of layer(s) of prepreg 1 on a winding shaft 4. As shown in FIG. 1B, in a second process step 20, following layer(s) of prepreg 2 are wound on winding shaft 4 by rotating the winding shaft 4 in a direction as shown. As shown in FIG. 1C, the third process step 30 shows the first layer(s) of prepreg 1 wound around the winding shaft 4 and the following layer(s) of prepreg 2 wound around first layer(s) 1, i.e., all layers of prepreg are wound.
German Patent Document 1923179 U relates to a tube-shaped shaft for the transmission of driving forces in a motor vehicle. The drive shaft of that document is made of a bending-resistant plastic material. In this case, reinforcing devices constructed as fibers or bands are arranged in a spiral-shaped manner in the shaft.
According to German Patent Document DE 1923179 U, it was known to a person skilled in the manufacturing of fiber composite materials that the reinforcing devices are impregnated with liquid plastic in a molding tool. As it cooled, the plastic material (epoxy or polyester resin) hardened and the reinforcing devices were integrated in the plastic material. The plastic material was used as a support for the reinforcing device. This document also teaches the insertion of woven material as a reinforcing device into the tube-shaped shaft. It uses no prepreg layers but woven-type dispersions in the tube wall (Page 3, Lines 22 to 24). The known solution has an insufficient damage tolerance stability which is not satisfactory for the helicopter construction.
German Patent Document DE 3822417 C2 relates to a method of producing a shaft for the transmission of torsional forces. The object of that document is the production of the shaft in a simple manner with respect to its manufacturing. The document teaches to produce the shaft by using the winding technique using fiber composite material. FIG. 3 of the reference shows a shaft which is produced from impregnated fiber mats or prepregs. In this case, the prepreg can be wound up spirally. This solution also does not yet have a satisfactory damage tolerance stability as required in helicopter construction.
The known prior art also has the disadvantage that the risk of wrinkling exists, for example, in the pressing mold (for example, vacuum bag). It is known to a person skilled with respect to fiber composite materials that the woven material layers undergo a setting (shrinkage) in the pressing mold which does not exclude a wrinkling. The wrinkling corresponds to a local deviation from the ideal fiber orientation in the woven material. This has a disadvantageous effect on the stiffness against torsion of the drive shaft.